(Corrects paragraph 7 to clarify patients on the placebo experienced a decline of 2.4, not 1.3)

By Natalie Grover

June 13 (Reuters) - A synthetic version of a medicine traditionally extracted from chili plant relieved knee pain among osteoarthritis patients for up to six months, data showed, bringing Centrexion Therapeutics a step closer to developing a safe and effective analgesic.

The drug, designed to be injected at the site of pain, is being developed by the privately-held company run by former Pfizer Inc chief executive Jeffrey Kindler.

Centrexion's drug, a man-made version of chili plant extract trans-capsaicin, is designed to work by inactivating local pain fibers transmitting signals to the brain.

The mid-stage trial tested two doses of the drug, CNTX-4975, against a placebo in 175 difficult-to-treat knee osteoarthritis patients who had failed or were unable to tolerate prior pain therapy.

Osteoarthritis affects about 14 million Americans. It is caused by the progressive breakdown and eventual loss of cartilage, and characterized by pain, swelling and decreased mobility of the affected joint.

Data showed the drug induced statistically significant pain relief as well as reduced knee stiffness and improved physical function at 24 weeks after a single injection.

Patients on the 1 milligram (mg) dose experienced a reduction of 3.8 on a scale measuring daily pain with walking, versus a decline of 2.4 for those on the placebo, the company said on Tuesday.

With exploding U.S. rates of abuse, overdose and addiction to opioids - a lethal family of drugs widely prescribed for pain - as well as side-effects seen with other pain treatments, developing an analgesic with little side-effects has become imperative.

The safety profile of CNTX-4975 was comparable to that of a placebo, chief medical officer Randall Stevens said, adding that the medicine is cleared out of the body 24 hours after it is injected.

"When you eat a hot chili meal, you're consuming about 25 mg of capsaicin. So the systemic exposure from the meal is actually higher," he told Reuters.

The Boston-based company, which is developing various non-opioid painkillers, expects to initiate a late-stage study later this year for CNTX-4975.

The drug is also being evaluated to treat patients with Morton's neuroma pain as well as canine osteoarthritis. (Reporting by Natalie Grover in Bengaluru; Editing by Arun Koyyur)